The present invention relates to an optical pickup. More particularly, it relates to an optical pickup for use in an optical recording/reproducing apparatus for reproducing information from an optical recording medium, recording information on an optical recording medium or erasing information from an optical recording medium.
Optical memory technology that uses, as a high density and large capacity recording medium, an optical disk with a pit pattern has been spreading in the application to, for example, digital audio disks, video disks, document file disks and data file disks. In the optical memory technology, information is recorded in and reproduced from an optical disk with high accuracy and high reliability by using finely restricted optical beams. This recording/reproducing operation wholly depends upon the optical system of a recording/reproducing apparatus. The basic functions of an optical pickup, that is, a principal part of the optical system, are roughly classified into the following: a collecting function to form a diffraction limited fine spot; a controlling function for focus control and tracking control of the optical system; and a detecting function for a pit signal. Each of these functions is realized by combining any of various optical systems and photoelectric conversion/detection methods in accordance with the purpose and the use.
In particular, an optical pickup using a hologram element has recently been developed in order to reduce the size and the thickness of the optical pickup. Also, in a conventional technique using a three-beam method for the tracking control, a diffraction element is used for diffracting a light beam into a main beam and a sub beam.
Now, an exemplified conventional technique will be described with reference to FIGS. 8, 9A and 9B. It is noted that, in xyz coordinates shown in these drawings, an identical direction is indicated by using identical axes of the coordinates.
FIG. 8 shows the architecture of a conventional optical pickup. The optical pickup of FIG. 8 includes a semiconductor laser 1, photodetectors 2 and 3, a diffraction element 4, a hologram element 5, a collimator lens 6 and an object lens 7, so as to perform a read operation for reading a pit pattern of an optical disk 8 and the like. This operation will now be described.
An outgoing light beam L0 from the semiconductor laser 1 passes through the diffraction element 4, so as to be divided into a pair of a main beam and a sub beam (not shown) to be used for detecting a tracking error signal. The main beam and the sub beam pass through the hologram element 5, are collected by the collimator lens 6 and enter the object lens 7. Then, the beams are collected on the optical disk 8 by the object lens 7.
A light beam reflected by the optical disk 8 enters the hologram element 5 through the above-described optical path in the reverse direction. At this point, ±1st-order diffracted light beams (L1 and L2 ) generated by the hologram element 5 respectively enter the photodetectors 2 and 3 to be detected. When the outputs of the photodetectors 2 and 3 are calculated, a focus error signal FE, a servo signal including a tracking error signal TE and an information signal can be obtained.
The structures of the hologram element 5 and the photodetectors 2 and 3 are shown in FIGS. 9A and 9B, respectively. FIGS. 9A and 9B respectively show the plane structures of the hologram element 5 and the photodetectors 2 and 3 taken along the negative direction of the z-axis of FIG. 8 (namely, a direction from the optical disk 8 toward the photodetectors 2 and 3 on the drawing).
The hologram element 5 is a Fresnel zone plate consisting of a single area with a hologram pattern as shown in FIG. 9A. FIG. 9B shows the positional relationship between an apparent light emitting point 1a of the semiconductor laser 1 and the photodetectors 2 and 3.
As shown in FIG. 9B, a detection face of the photodetector 2 is divided into areas 2a, 2b, 2c, 2d and 2e. Also, a detection face of the photodetector 3 is divided into areas 3a, 3b, 3c, 3d and 3e. 
As shown in FIG. 8, the diffracted light beams L1 and L2 obtained by the hologram element 5 respectively enter the photodetectors 2 and 3. In FIG. 9B, cross-sections of light beams on the photodetectors 2 and 3 are shown as circles L1a, L1b, L1c, L2a, L2b and L2c. In this case, the cross-sections L1b and L2b correspond to spots of the main beam, and the cross-sections L1a, L1c, L2a and L2c correspond to spots of the sub beam.
Since the hologram element 5 is a Fresnel zone plate, the diffracted light beam L1 is converged on a point in front of the apparent light emitting point 1a of the semiconductor laser 1 (namely, on a point away from the apparent light emitting point 1a in the positive direction of the z-axis and in the perpendicular direction to the face of the drawing). Also, the diffracted light beam L2 is converged on a point behind of the apparent light emitting point 1a (namely, on a point away from the apparent light emitting point 1a in the negative direction of the z-axis).
The focus error signal FE is detected by a known SSD (spot size detection) method utilizing this difference in the convergence points. In other words, the focus error signal FE is obtained through calculation of the following equation 1, wherein the reference numerals of the respective detection areas of the photodetectors 2 and 3 are used to indicate output values of the corresponding detection areas:FE=(2c−2b−2d)−(3c−3b−3d)  Equation 1
On the other hand, the tracking error signal TE is detected by a known three-beam method. In other words, the tracking error signal TE is obtained through calculation of the following equation 2, wherein the reference numerals of the respective detection areas are used to indicate output values of the corresponding detection areas:TE=(2a+3a)−(2e+3e)  Equation 2
The conventional optical pickup, however, is difficult to apply to reproducing or recording operations for various optical recording mediums with different physical formats, such as CDs, DVD-ROMs and DVD-RAMs. As a countermeasure against this problem, Japanese Laid-Open Patent Publication No. 2001-229573 discloses an optical pickup applicable to reproducing or recording operations for various optical recording mediums with different physical formats, which is insufficient in the reliability. Specifically, in the optical pickup disclosed in this publication, the shift of a light spot is largely restricted, and for example, the shift of merely approximately 0.1 mm is allowable. Therefore, there arises another problem that it is actually not easy to mass-produce optical pickups with the shift at such a low level.
Furthermore, according to Japanese Laid-Open Patent Publication No. 2001-229573, a photodetector, a hologram element and the like are disposed separately from a semiconductor laser (light source). Therefore, it is very difficult to align respective elements of the optical system with a small shift. Also, since the semiconductor laser (light source) is separately disposed, this pickup is disadvantageously easily affected by vibration.